The invention relates to a method of controlling an internal combustion engine-generator unit.
Normally, an internal combustion engine that is to be used as a generator drive is delivered by the manufacturer to the final customer without a clutch and a generator, which are mounted only at the final customer's facility. In order to ensure a constant nominal frequency for feeding current into an electrical network, the internal combustion engine is operated in a rotational speed control loop, in which the rotational speed of the crankshaft is detected as the controlled variable and is compared with a desired rotational speed (the “command variable”). The resulting control deviation is converted by way of a rotational speed controller to a manipulated variable for the internal-combustion engine, such as a desired injection quantity.
Since the manufacturer often does not have any proven data concerning the clutch characteristics and the moment of inertia of the generator prior to the delivery of the internal combustion engine, the electronic control unit is delivered with an enduring set of controller parameters (a so-called standard parameter set). One problem of a rotational speed control loop is that torsional oscillations, which are superimposed on the controlled variable, may be intensified by the rotational speed controller. The low frequency oscillations (such as the torsional oscillations of the 0.5th and 1st order) caused by the internal combustion engine are particularly critical. When starting the internal combustion engine-generator unit, as a result of the intensification of the rotational speed controller, the amplitudes of the torsional oscillations may become so high that a rotational limit speed is exceeded and the internal combustion engine is turned off. In practice, this means that trained personnel at the final customer's facility must adapt the standard parameter set to the on-site conditions, at high expenditures and costs.
The instability problem is countered by a rotational speed filter in the feedback branch of the rotational speed control loop. Such a rotational speed filter is disclosed, for example, in European Patent Document EP 0 059 585 B1. In this filter, the tooth period of a shaft is detected during a working cycle of the internal combustion engine (two rotations of the crankshaft, corresponding to 720 degrees). Subsequently, a filtered tooth period is computed from these tooth periods by forming the arithmetic mean. This filtered tooth period is updated after each working cycle and corresponds to a rotational speed value, which is then used to control the internal combustion engine. However, in the case of this two rotation filter, it is a problem that stable behavior of the drive system is accompanied by a deterioration of the design load behavior. A poor design load behavior may mean that, under certain circumstances the legally required design load criteria are no longer met.
It is an object of the present invention to provide a control for an internal combustion engine-generator unit which ensures a fast and reliable design load behavior.
This and other objections and advantages are achieved by the control method according to the invention, in which the filtered rotational speed is monitored with respect to rotational speed oscillations. When a rotational speed oscillation is determined, its frequency is compared with a first limit value, and as a function of the comparison, a first or second mode is set. That is, the first mode is set at a frequency which is greater than the first limit value, and the second mode is set at a frequency which is smaller than the first limit value. As noted below, in order to eliminate the oscillation the filter is altered (in a manner discussed below) in the first mode, and parameters of the rotational speed controller are adapted in the second mode.
The invention is based on the recognition that the natural frequency of such an internal combustion engine-generator unit typically is in a range of from 10 to 25 Hz.
In the first mode, the filter is altered in that a new crankshaft angle (that is the rotational angle over which the rotational speed is averaged) is used for the filtering of the actual rotational speed. The new crankshaft angle is determined by means of a characteristic curve, as a function of the frequency to be filtered out. For example, a 25 Hz oscillation is filtered out when the filter averages the actual rotational speed through a crankshaft rotation of 360 degrees. Another measure consists of reducing the P-portion and/or the D-portion of the rotational speed controller. For this purpose, a proportional action factor and/or a derivative action time is reduced.
In the second mode, the frequency is compared with a second limit value. At a frequency lower than the second limit value, a P-portion and an I-portion are adapted as parameters of the rotational speed controller. The second limit value, for example, has a value of 7 Hz. The adaptation takes place in that, after each value change, it is checked again whether rotational speed oscillations are still present.
The invention and its further developments make it possible to design the standard parameter set of the rotational speed controller for a rotationally rigid internal combustion engine-generator unit (that is, for a system without a clutch). Rotationally rigid systems are ideal systems for designing the rotational speed controller because they permit the use of a fast rotational speed controller (high proportional action factor, long derivative action time). As a result, a fast design load behavior is ensured.
By the introduction of the first limit value (for example 10 Hz), an oscillation problem can be identified. In the case of a frequency greater than the first limit value, the cause is the clutch. In the case of a frequency lower than the first limit value, the cause is a very high moment of inertia of the generator. By altering the rotational speed filter or adapting parameters of the rotational speed controller, the advantage is obtained that the system stabilizes itself. In this case, the dynamics of the system are reduced only as much as necessary so that the internal combustion engine-generator unit operates reliably. Since the standard parameter set is adapted automatically, the final customer has a freer choice of clutches. Because the need for an additional on-site adaptation is eliminated, customer servicing costs are reduced.
Another advantage is that the necessary number of standard parameter sets is lower, since no clutch-specific and/or generator specific data sets have to be created.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.